Please forgive my description of this. On the outside of our side sliding windows is a thin black rubber infill piece. It covers and seals over the fixings. Does anyone know where I can get some of this seal from. The seal we have at the moment seems to have gone brittle and shrunk.
Seals Direct is the best you will find for this kind of thing... ask them nicely and then will even send a small piece of likely sizes for you to try if you are unsure.
They have even been known to give a discount if you mention this site!!! ( I think) Screw Cover Strip is the stuff you want... (I think)